Sam is an interesting character indeed, in that he is modeled after the author himself, and therefore has many interesting character traits such as friendliness, charity, and intelligence.
Just like Paul Fleischman went "out of my way to start friendly conversations with people," Sam of chapter six does the same, calling it his "occupation" to get people to simply smile at him. Friendliness, then, is a major part of Sam's character. He is always trying to fix the social network of the area with his purposeful geniality. Sam also loves a challenge of someone who may not readily smile at him.
Secondly, Sam is charitable. I suppose his friendliness in itself could be seen as a sort of charity; however, Sam also has an actual career of helping non-profit organizations, especially ones that focus on peace in world government. In addition, Sam's gardening can be seen as a form of beautifying charity as well.
Finally, Sam is intelligent. He is always talking about the history (or the etymology) of words, for that is his favorite thing to study. He is happy to share this knowledge with others as well.
I suppose I could go on further and add a couple more static character traits such as Sam is old and Jewish and rich and good with the soil as well. However, those just don't seem as interesting as the first three I mentioned.